The present invention relates to multichannel communications systems, and more particularly to multichannel communications systems where the modulation and demodulation of the signals to be communicated is accomplished by direct implementation of the mathematical equations characterizing those signals.
Many systems have been proposed for transmitting and receiving stereophonic signals in the AM frequency band. Several of these schemes have proposed amplitude modulating the two program signals onto differently phased carriers. These differently phased carriers are linearly combined to form the composite modulated signal which is to be transmitted. Some of the proposed systems have set the phase angle between the two carrier signals at approximately 90.degree., thus providing a standard quadrature modulating scheme. Other systems have been proposed where the phase angle between the two carriers is reduced to less than 90.degree.. A system of this latter sort is described in the co-pending patent application of Leitch, U.S. Ser. No. 812,657, filed July 5, 1977.
The systems which have been devised to generate and transmit these composite modulated signals have generally utilized conventional AM modulation techniques. Thus, in these systems each of the program signals is modulated onto a separate carrier signal, and the carrier signals are then added together to create the composite modulated signal. In order to transmit this signal with a standard AM transmitter, this composite modulated signal is separated into phase and amplitude portions. More specifically, the composite modulated signal is hard-limited to provide a constant amplitude, phase modulated RF signal. This RF signal is directed to the RF input of the AM transmitter. The composite modulated signal is also envelope detected to provide an audio frequency signal having an amplitude defining the envelope of the composite modulated signal. This audio signal is directed to the audio input of the conventional AM transmitter. The transmitter amplifies the RF and audio signals and then recombines them to form a high-level composite modulated signal for transmission. These previous modulators therefore utilize three separate steps: modulation to form a low level composite modulated signal; demodulation to separate the composite modulated signal into audio and RF portions; and modulation to form a high level composite modulated signal for transmission.
One problem encountered in the implementation of these systems resides in the inequality of the time delays in the audio and RF channels of a conventional AM transmitter. It is possible for the time delay of the signals in the audio (envelope) channel to be much larger than the time delay in the RF (phase) channel. In this event, it would be necessary to insert an appropriate delay in the RF channel, so that the phase and amplitude information of the signal transmitted by the transmitter were appropriately synchronized. Implementation of the required radio frequency delays, however, is not easily accomplished.
The receivers which have been proposed for receiving signals modulated in this fashion generally utilize conventional synchronous demodulation methods. In these receivers, a phase-locked-loop is generally used to recover the carrier component of the modulated signal, with this carrier component then being utilized in conjunction with product detectors to synchronously recover the program signals from the composite modulated signal.